To address the limitations of centralized shared storage for cloud computing, we are building Lithium, a distributed storage system designed specifically for virtualization workloads running in large-scale data centers and clouds. Lithium aims to be scalable, highly available, and compatible with commodity hardware and existing application software. The design of Lithium borrows ideas and techniques originating from research into Byzantine Fault Tolerance systems and popularized by distributed version control software, and demonstrates their practical applicability to the performancesensitive problem of VM hosting. To our initial surprise, we have found that seemingly expensive techniques such as versioned storage and incremental hashing can lead to a system that is not only more robust to data corruption and host failures, but also often faster than naïve approaches and, for a relatively small cluster of just eight hosts, performs well compared with an enterprise-class Fibre Channel disk array.